Testability is a design characteristic of electronic circuits that, among other factors, determines the cost of testing the circuit. Two important testability attributes are controllability and observability. Controllability is the ability to establish a specific signal value at each node in a circuit by setting values on inputs to the circuit. Observability is the ability to determine the signal value at any node in a circuit by controlling inputs to the circuit and observing outputs from the circuit. Oscillators, clock generators, feedback circuits and decoders are typically very difficult to control.
What is particularly difficult to achieve is testing of semiconductor circuits having an oscillator that is not synchronized to the test system. For such circuits, a tester has difficulty recognizing test vectors on a device under test. Contemporary microchips typically have hundreds or thousands of test vectors and, unless the oscillator of a circuit under test is synchronized to the frequency of the tester, drift or slippage of test vectors occurs. Drift or slippage of a test pattern is a timing error in which the timing of zero and one logic levels generated by the circuit under test differs from the timing expected by the tester so that the tester and circuit under test are not synchronized.
What is needed is a technique for precisely controlling timing signals of a circuit for testing purposes.